Electrophotographic image receiving element with means to space said element from an image bearing surface during image transfer



July 7, 1 970 E E R EI'AI 3,519,819

ELECTROPHOTOGRAPHIG IMAGE RECEIVING ELEMENT WITH MEANS To-sPAcE SAIDELEMENT FROM AN IMAGE BEARING SURFACE DURING IMAGE TRANSFER Filed Oct.9. 1967 \Y /MA 65- BEAR/N6 ELEMENT -A/R GAP PART/CULATE sPAcv/va MEANS\Q P/GME/VT B/NDEI? S UPPO/P T IMAGE-BEARING ELEMENT l\ A T 23 SPHER/CALSPACl/VG MEANS P/GME/VT 24 I I BINDER I SUPPORT EUGENE GRAMZA GENE H.ROB/IVSO/V A INVENTORS ATTORNE Y United States Patent 3,519,819ELECTROPI-IOTOGRAPHIC IMAGE RECEIVING ELEMENT WITH MEANS T0 SPACE SAIDELE- MENT FROM AN IMAGE BEARING SURFACE DURING IMAGE TRANSFER Eugene P.Gramza and Gene H. Robinson, Rochester,

N.Y., assignors to Eastman Kodak Company, Rochester, N.Y., a corporationof New Jersey Filed Oct. 9, 1967, Ser. No. 673,544 Int. Cl. G03g 5/02U.S. Cl. 250-65 18 Claims ABSTRACT OF THE DISCLOSURE Receiving elementshaving a regulated surface roughness are capable of producingelectrographic images of improved quality over elements having smoothsurfaces.

This invention relates to image-receiving elements and the process forusing them, and particularly to receiving elements for use inelectrostatic processes. I

The process of xerography as disclosed by Carlson in U.S. 2,297,691employs an electrophotographic element comprising a support materialbearing a coating of a normally insulating material whose electricalresistance varies with the amount of incident actinic radiation itreceives during an imagewise exposure. The element, commonly termed aphotoconductive element, is first given a uniform surface chargegenerally in the dark after a suitable period of dark adaptation. It isthen exposed to a pattern of actinic radiation which has the effect ofdifferentially reducing the potential of the surface charge inaccordance with the relative energy contained in various parts of theradiation pattern. The differential surface charge or electrostaticlatent image remaining on the electrophotographic element is thentransferred to an electrophotographic receiving element. The transferoperation is Well known in the art and is typically described in U.S.Pat. 2,825,814. Usually the receiving element is a white paper coatedWith a thin layer of an insulating polymeric material in such a manneras to provide a smooth surface.

The transfer is generally carried out by contacting the insulatingsurface of the exposed photoconductive element with the surface of theelectrophotographic receiving element. An electric field is establishedbetween these surfaces and the electrostatic charge is transferred tothe surface of the receiving element. Since the surface is coated withan insulating polymeric material, it is trapped there. The transferredlatent image is then made visible are relatively dilficult to obtain byusing normal mechanical methods. If the space is too small, there can besubstantial transfer of charge in the background areas thus frequentlyresulting in a mottled background. If the space is too large, thenlittle or no charge will be transferred. In U.S. Pat. 2,825,814 Walkupdescribes a method for maintaining this spacing by grinding a plastic orresin to a relatively uniform particle size in the order of the size ofspacing desired between the two members. A small quantity of thepowdered material is dusted onto one of the smooth surfaces. The :secondsurface is then placed on top of the dusted surface and the transferoperation carried out. The disadvantage with this type of operation isthat the dusted particles tend to adhere to both surfaces after thetransfer operation is complete and after the surfaces are no longer incontact. Also, when the latent image is subsequently developed by theapplication of a toner, the final image often contains blotches due tothe presence of the small particles used to maintain the spacing.Oftentimes when the two elements are separated after the transfer of alatent or developed image, theparticles move slightly if the utmost careis not taken. As a result of this movement the transferred image isfrequently of inferior quality.

It is therefore an object of this invention to provide a novel imagereceiving element which is capable of receiving latent and developedimages having good quality from an image-bearing element.

It is another object of this invention to provide a novel process fortransferring latent electrostatic images from the insulated surface ofan electrostatic image-bearing element to the insulated surface of areceiving element.

It is a further object of this invention to provide a novel process fortransferring developed electrostatic images from the insulated surfaceof an electrostatic image-bearing element to the insulated surface of areceiving element.

These and other objects of this invention are accomplished by animage-receiving element which has a controlled surface roughness. Acorrelation has been found to exist between the texture of the surfaceof the receiving element asmeasured in terms of Sheffield SmoothnessValue and the quality of the transferred image obtained. Best resultsare obtained if the Sheffield Smoothness of the elements of theinvention are about 80 to about 180 and preferably 90 to 160.

The Sheflield Smoothness Value is a measure of paper smoothness whichconforms to TAPPI Standard No. T-479SM48. The equipment for making themeasurement is made by the Sheffield Corp, Dayton, Ohio.

Briefly, the components of the equipment are: (1) .a

by contacting the surface with a suitable electroscopic markingmaterial. Such marking material or toner, whether contained in aninsulating liquid or on a dry carrier, can be deposited on the receivingelement either in the areas where there is an electrostatic charge or inthe areas where the charge is absent.

Alternatively, prior to transfer, the electrostatic latent image can bedeveloped directly on the photoconductive element in the same manner setforth above. The developed image can be transferred to the receivingelement by contacting the two surfaces and applying an electricalpotential between them.

When the deposited marking material is on the surface of the receivingelement representing the developed image, it can then be fixed there byknown means such as heat, pressure, solvent vapor or the like.

During the transfer operation, in order to obtain good quality,background-free reproductions it is desirable to maintain a minute airgap of a few microns between the smooth surface of the receiving elementand the surface of the photoconductive element. Such small spacingsprecision device in which the paper sample is held against a smoothglass plate under an accurately weighted precision machined head throughwhich regulated airflows; and (2) a Modular Precisionaire Instrumentwhich measures the flow of air across the surface of the paper sample.Data are read in numerical units from 0 (smooth) to 400 (rough).

While the elements of this invention are useful in any process whereinit is desired to transfer an electrostatic image, they are particularlyuseful in the transfer of images in electrophotographic systems such asxerography. According to this invention, a member bearing anelectrostatic latent image such as a xerographic photoconductive elementis brought into contact with the face of a receiving element. Byproviding a receiving element which has a surface roughness Within theabove indicated limits, the size of the air gap can be very closelycontrolled.

The receiving elements of controlled surface roughness of this inventionare prepared by coating a suitable substrate such as paper with a thinlayer containing an electrically insulating, solid, film-formingpolymeric binder and particulate spacing means randomly dispersedthroughout the layer and embedded therein. The spacing means comprise aplurality of substantially inert filler particles such as bariumsulfate, zinc oxide, titanium dioxide, zinc carbonate, calciumcarbonate, polystyrene beads, glass beads and the like. Almost anyinorganic or organic material is suitable for such use provided it has asoftening point or melting point substantially above that at which thetransfer or developing operations are performed and is inert chemically,physically, electrically, etc., under the conditions used. Each of theseparticles is embedded in the polymeric binder layer in such a mannerthat a portion of each particle protrudes above the surface of thelayer. The layer itself is generally 3-6 microns thick and preferabl 4to 5 microns. However, layers having a thickness from 1 to about micronscan be employed if desired.

The size of the spacing means employed determines the size of the airgap and thus the separation between the surfaces of the photoconductingand receiving elements. Since a portion of the filler particles isembedded in the surface of the layer and a portion protrudes above thesurface of the layer, only the size of the protruding portion isconsidered in the determination of the gap width. By the term effectivediameter is meant the greatest linear dimension of the filler particle.The filler particles can have a wide variety of shapes. For example, theparticles can be spherical, polyhedral, conical, cylindrical, etc. Ineach instance however, the effective diameter can range from 5-35microns and preferably from 7-25 microns. Particles having larger andsmaller effective diameters are also usable such as 2-50 microns.

The air gap between the surfaces typically is 2 to 30 microns andpreferably 3-20 micons and thus, this much of the spacing meansprotrudes above the surface of hte receiving element. Air gaps of 1-40microns are acceptable in many applications and in these instances thisportion of the spacing means extends above the surface.

The electrophotographic receiving elements of this invention can containa substance which will give the element a desired color such as white,green, red, blue, black, etc. This substance is generally a pigmentwhich is present in the coated layer in an amount sufficient to impart auniform pigmentation to the viewer. However, if the color of the elementis not critical, then the presence of a coloring substance is notnecessary. If the desired color is white, then the coloring substancecan be any white pigment such as particles of barium sulfate, zincoxide, titanium dioxide, zinc carbonate, calcium carbonate or bentonite.The size of the pigmenting particles can vary from 0.1 to 1.0 micron andusually from 0.2 to 0.5 micron. Ideally, particles having an effectivediameter of 0.3 micron are preferred.

The binder can be any electrically insulating, solid, film-formingpolymer. Such polymers include (a) polyvinylbutyral,

(b) poly(bis-phenol A) carbonate,

(0) polystyrene,

(d) a polyester of terephthalic acid and a mixture of ethylene glycol (1part by weight) and 2,2-bis[4-(B-hydroxyethoxy)phenyl]propane (9 partsby weight),

(e) polyvinylformal,

(f) a copolymer of vinylchloride and acrylonitrile,

(g) a copolymer of vinylchloride and vinyl acetate, and

(h) poly(4,4-[2-norbornylidene]-diphenyl carbonate).

A wide variety of substrates or supports can be used for the presentreceiving elements, but the preferred one is paper. The paper can have aclay subbing on one side but beneficial results are obtained if bothsides are subbed. If both sides are subbed, then the paper absorbs lessof the developer and drying time and costs are substantially reduced. Itis also preferable to nip-size one side of the paper with a conductingresin such as polyvinylbenzyl-ammonium chloride. While this treatment ofthe paper is not absolutely necessary, if it is omitted oftentimes thedeveloped image will print through onto the back of the paper. Othersuitable substrates include polyethylene-coated paper, glass,polystyrene, cellulose acetate and polyethylene terephthalate.

Thus far, the invention has been set forth in general terms. It will nowbe described illustratively in terms of the drawings.

FIG. 1 is a diagrammatic view of an electrophotographic receivingelement according to one embodiment of this invention.

FIG. 2 is a diagrammatic view of an electrophotographic receivingelement according to another embodiment of this invention. In bothfigures the illustrations are diagrammataic, largely because certain keymembers and certain important spacings are extremely thin and small.

In FIG. 1 is shown an image-bearing element 11 having either a latentelectrostatic image or a developed image on the surface thereof. Spacedtherefrom by a minute air gap 12 of a few microns is a receiving elementto which the image on the surface of the image-bearing element is to betransferred. The receiving element itself contains a support 14 such aspaper. Coated on the support is a thin layer of an electricallyinsulating, solid, film-forming, polymeric binder 15. Dispersedsubstantially uniformly throughout the layer are very small pigmentingparticles 16. The pigmenting particles are not necessary unless it isdesired to impart a particular color to the receiving element. The layeralso contains particulate spacing means 13. This is generally aplurality of substantially inert filler particles randomly dispersedthroughout the layer and embedded therein. It is the protrusion of theseparticles above the layer which imparts roughness or an uneven textureto the surface. The size of the individual particles controls the widthof the minute air gap. Since a portion of the particle is embedded inthe binder, it is firmly held in position during the image transferprocess. Preferably, at least 4 microns of each of the particles isembedded in the surface to ensure that they will not become dislodgedduring the transfer operation. Thus, upon separation of the elements agood quality image is obtained with no background density and n0blotching. The particles are, of course, larger than the thickness ofthe layer.

In order for the transfer operation to occur, the imagebearing elementand the receiving element are brought into close proximity of each otherin a face-to-face relationship. The two are separated only by a spaceequal to the height which the spacing means protrudes above the surfaceof the layer. The only points of contact are where the spacing meanstouches the image-bearing element. both elements are mounted onconducting supports (not shown). A source of electrical potential isapplied to the conducting supports to establish an electric fieldbetween them. The developed or latent electrostatic image on the surfaceof the image-bearing element is caused to leave and be deposited on thesurface of the receiving element as a result of the electric field.Since the layer of the receiving sheet contains a binder which is aninsulator, the charge representing the image is trapped there. The widthof the air gap is 2 to about 30 microns. Best transfers are obtained ifthe air gap is 3-20 microns, however, gaps from 1 to 40 microns are alsooperable.

FIG. 2 is similar to FIG. 1 except that the spacing means 23 is depictedas being spherical particles rather than irregular shaped particles. Theremaining components are the same; namely, image-bearing element 21,minute air gap 22, receiving element support 24, polymeric binder 25 andpigmenting particles 26.

In preparing the receiving elements of this invention, the averagedistance between the spacing particles is typically 50 to 200 microns,with about microns be ing optimum for most transfers but averagespacings of 25 to 500 microns are also operable. The elements are usefulin any process where it is desired to transfer an electrostatic imagefrom the surface of a member to the surface of a receiving element. Theelements are particularly useful, in xerographic processes where animage is reproduced by simultaneously charging and exposing aphoto-conductive element. Such a process is morefully described in Ser.No. 665,911, by G. H. Robinson, filed Sept. 6, 1967, now abandoned. a

The invention is further illustrated by the following examples whichinclude preferred embodiments thereof.

' EXAMPLE 1 The following formulation is used to prepare aninsulatingmaterial suitable for coating:

75 gm.polyrneric binder of a ter-polymer of vinyl butyral (88%), vinylalcohol (943%), vinyl acetate (2 /2%), having an approximate molecularweightof 50,000 (such as Butvar B-76, made by Shawinigan Resins Corp.)

22.75 gm.pigmenting particles of titanium dioxide having an averageparticle size of 0.3 micron (Titanox RA manufactured by TitaniumPigments Corp.)

2.25 gm.spacing filler particles of titanium dioxide having an averageeffective diameter of 7 to 25 microns (Titanox' TG manufactured byTitanium Pigments 'Corp.)

670 gm.-toluene This mixture is milled for 3 hours and 47 minutes in aKadymill at 15 percent solids..It is then coated at a dry weight of 0.55g. per square foot to produce a dry thickness of 4 to 5 microns on apaper which has a clay subbing on one side only. The insulating layer iscoated on the subbed side. The texture of the insulator-coated side ofthe paper as indicated by the 'Shefiield Smoothness Value is 140. Anelectrostatic latent image-bearing photoconductive element is placed ina face-to-face relationship in close proximity to the receiving elementin such a manner that thereis a 20-micron air gap between the surfacesof the two elements. The spacing .in this air gap is controlled by theportion of the spacing titaniumdioxide particles which extend above thesurface of the receiving element. A potential of approximately 1500volts DC, with respect to ground, is applied for approximately 2 secondsto the conducting layer of the photo-conductive element; the conductingsupport of the receiving element being grounded. At the end of thetransfer operation, the potential is removed and the various, membersseparated. The receiving member, bearing the electrostatic latent imagecorresponding to that borne by the surface of,the

photoconductive element, is developed by immersion in a. rpositive-polarity liquid developer. The resulting image is apostive-appearing reproduction displaying dense, sharp, black characterswith a uniform low density background. The back side of the developedreceiving element displays a negative-appearing image of the developedimage on the front of the sheet.

EXAMPLE 2 An insulator-coated receiver paper is prepared exactly asdescribed in Example 1. The uncoated side of the paper is nip sized withan 8-percent solution of a conducting resin of polyvinylbenzyl-ammoniumchloride in water. The Sheffield Smoothness Value of the insulatingsurface. is 140. A print is made on this paper in the same manner asdescribed in Example 1. The resultant image displays dense, sharp, blackcharacters with a clean white background. No toner deposition isdetected on the rear side of the sheet.

EXAMPLE 3 An insulator-coated receiver paper is prepared according to.the formulation set forth in Example 1, except that 25 g. of titaniumdioxide pigmenting particles are used and no titanium dioxide spacingparticles are used. The Sheflield Smoothness Value of this material is40. A print is made on this paper according to Example 1.

The resultant image is low in density with an extremely high andirregular background density. This type of insulator-coated paper isvery similar to the prior art papers, however, no spacing means,mechanical or otherwise, is used. It produces an unacceptable finalprint.

EXAMPLE 4 For purposes of comparison, Example 3 is repeated except thata small amount of polystyrene beads having an average diameter of 20microns is dusted onto the surface of the photoconductive element assuggested in US. Pat. No. 2,825,814. These particles are used tomaintain an air gap of 20 microns between the surface of thephotoconductive element and the surface of the receiving element. Aprint is made in the usual manner as described in Example 1. Theresultant image is good except that there are blotches on the paper.These blotches occur since the spacing particles are dusted on thesurface of the element instead of being embedded in the surface of thereceiving element.

EXAMPLE 5 This example is the same as Example 2, except that a paper isused which has a clay subbing on both sides. The resultant imageobtained after developing is comparable to that described in Example 2.However, the amount of developer absorbed into the paper support isconsiderably less and the drying time of the final print is reducedgreatly.

EXAMPLE 6' An insulator-coated receiver paper is prepared according tothe formula of Example 1, except that larger titanium dioxide spacingparticles are used so that the Sheffield Smoothness of the paper is 250.The imageforming procedure of Example 1 is followed and in this instanceno image is transferred from the surface of the photoconductive elementto the surface of the receiving paper because the air gap provided istoo large for the charge to be transferred.

EXAMPLE 7 The purpose of this example is to show that the samecharacteristics which make for optimum charge transfer are alsoextremely desirable in a receiver sheet for a transfer of liquiddeveloped xerographic images. This type of transfer of a liquiddeveloped image is particularly useful in the production of 3-colorsubtractive prints by multiple transfer to a single receiving sheet. Aphotoconductive element bearing an electrostatic latent image isdeveloped by pumping a liquid dispersion of cyan toner through adeveloping electrode as the photoconductor is moved across theelectrode. The resulting visible wet image is overlaid with a sheet ofcoated reoeiving paper of the type described in Example 2 above and thecombination is passed once rapidly beneath a negative coronawire. Thisoperation transfers essentially of the image to the receiving sheet,leaving but a detectable residue on the photoconductor. The quality ofthe image is very good. The same technique using a lithographic gradepress paper gives a very incomplete transfer and blotchy image. Theabove-described electrophotographic receiving elements are not limitedin their use to receiving images from certain types of photoconductiveelements; rather, the elements of this invention arecapable of receivingimages from any type of photoconductive elements including thosecontaining organic including organ0-metallic photoconductors as well asinorganic photoconductors.

\ The invention has been described in detail with particular referenceto preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected Within the spirit and scopeof the invention as described hereinbefore and as defined in theappended claims.

What is claimed is:

1. An electrophotographic image receiving element comprising a papersupport having coated thereon a layer having a thickness of 4 to 5microns comprising:

(a) an electrically insulating, solid, film-forming polymeric binder,

(b) a particulate pigment substantially uniformly dispersed throughoutsaid layer in an amount sufficient to impart whiteness thereto, theaverage effective diameter of said particulate pigment being 0.3 micronand (c) particulate spacing means comprising a plurality ofsubstantially inert filler particles embedded in the polymeric binder insuch a manner that said filler particles protrude 3 to 20 microns abovethe surface of said layer, said particles having an effective diameterof 7 to 25 microns and being randomly dispersed throughout said layerwith an average spacing between particles of 100 microns.

2. The element of claim 1 wherein the pigment is selected from the groupconsisting of barium sulfate, zinc oxide, titanium dioxide, zinccarbonate, calcium carbonate and bentonite.

3. The element of claim 1 wherein the filler particles are selected fromthe group consisting of barium sulfate, zinc oxide, titanium dioxide,zinc carbonate, calcium carbonate, polystyrene beads and glass beads.

4. The element of claim 1 wherein the polymeric binder is selected fromthe group consisting of (a) polyvinyl butyral,

(b) poly(bis phenol A) carbonate,

(0) polystyrene,

(d) a polyester of terephthalic acid and a mixture of ethylene glycoland 2,2-bis[4-(;3 hydroxyethoxy) phenyl] propane,

(e) polyvinylformal,

(f) a copolymer of vinylchloride and acrylonitrile,

(g) a copolymer of vinylchloride and vinyl acetate,

and

(h) poly(4,4'-[2-norbornylidene]-diphenyl carbonate).

5. The element of claim 1 wherein the binder is a polymer of vinylbutyral, the pigment is titanium dioxide and the filler particles aretitanium dioxide.

6. An electrophotographic image-receiving element comprising a supporthaving coated thereon a layer comprising:

(a) an electrically insulating, solid, film-forming polymeric binder and(b) particulate spacing means comprising a plurality of substantiallyinert filler particles embedded in the polymeric binder in such a mannerthat said filler particles protrude from about 1 to about 40 micronsabove the surface of said layer, said particles having an effectivediameter from about 2 to about 50 microns and being randomly dispersedthroughout said layer with an average spacing between particles rangingfrom about 25 to about 500 microns.

7. An electrophotographic receiving element comprising a support havingcoated thereon a layer comprising:

(a) an electrically insulating, solid, film-forming polymeric binder,

(b) a particulate pigment substantially uniformly dispersed throughoutsaid layer, the average effective diameter of said particulate pigmentranging from about 0.1 to about 1 micron and (c) particulate spacingmeans comprising a plurality of substantially inert filler particlesembedded in the polymeric binder in such a manner that said fillerparticles protrude from about 1 to about 40 microns above the surface ofsaid layer, said particles having an effective diameter from about 2 toabout 50 microns and being randomly dispersed throughout said layer withan average spacing between particles ranging from about 25 to about 500microns.

8. The element of claim 7 wherein the layer has a thickness from 1 toabout 10 microns.

9. The element of claim 7 wherein the support is paper.

10. The element of claim 9 wherein the particulate spacing means impartsa Sheffield Smoothness Value to the coated paper from about to about180.

, 11. The element of claim 9 wherein the paper support is sized on bothsides by a clay coating.

12. The element of claim 9 wherein the paper support is sized on bothsides by a clay coating and subbed on one side by a conducting resin.

13. A process for transferring an electrostatic image comprising:

(a) providing an element having an electrostatic image-bearing surface,

(b) providing an electrostatic image receiving element comprising asupport having coated thereon a layer comprising 1) an electricallyinsulating, solid, film-forming polymeric binder,

(2) a particulate pigment substantially uniformly dispersed throughoutsaid layer, the average effective diameter of said particulate pigmentranging from about 0.1 to about 1 micron and (3) particulate spacingmeans comprising a plurality of substantially inert filler particlesembedded in the polymeric binder in such a manner that said fillerparticles protrude from about 1 to about 4.0 microns above the surfaceof said layer, said particles having an effective diameter from about 2to about 50' microns and being randomly dispersed throughout said layerwith an average spacing between particles ranging from about 25 to about500 microns,

(c) positioning said electrostatic image-bearing surface and the surfaceof said receiving element in close proximity to each other in aface-to-face relationship so that the filler particles embedded in saidreceiving element physically contact the electrostatic image-bearingsurface thus creating a minute air gap between the two surfaces, and

(d) applying an electric field between the surface of the receivingelement and the electrostatic imagebearing surface causing theelectrostatic image to transfer to the surface of the receiving element.

14. The process of claim 13 wherein the inert filler particles areselected from the group consisting of barium sulfate, zinc oxide,titanium dioxide, zinc carbonate, calcium carbonate, polystyrene beadsand glass beads.

15. The process of claim 13 wherein the element having the electrostaticimage-bearing surface is a photoconductive element.

16. A process for transferring a developed electrostatic imagecomprising:

(a) providing an element having a developed electrostatic image-bearingsurface,

(b) providing an electrostatic image receiving element comprising asupport having coated thereon a layer comprising (1) an electricallyinsulating, solid, film-forming polymeric binder,

(2) a particulate pigment substantially uniformly dispersed throughoutsaid layer, the average effective diameter of said particulate pigmentranging from about 0.1 to about 1 micron and (3) particulate spacingmeans comprising a plurality of substantially inert filler particlesembedded in the polymeric binder in such a manner that said fillerparticles protrude from about 1 to about 40 microns above the surface ofsaid layer, said particles having an effective diameter from about 2 toabout 50 microns and being randomly dispersed throughout said layer withan average spacing between particles ranging from about 25 to about 500microns,

(c) positioning said developed image-bearing surface 18. The process ofclaim 16 wherein the element havand the surface of said receivingelement in close ing the electrostatic image-bearing surface is aphotoproximity to each other in a face-to-face relationconductiveelement. ship so that the filler particles embedded in said red ceivingelement physically contact the developed 5 References image-bearingsurface thus creating a minute air UNITED STATES PATENTS gap between thetwo surfaces and (d) applying an electric field between the surface of uthe receiving element and the developed image-bear- 3079253 2/1963 Graiging surface causing the developed image to trans- 1O fer to the surfaceof the receiving element. RALPH G. NILSON, Primary Examiner 17. Theprocess of claim 16 wherein the inert filler C E CHURCH AssistantExaminer particles are selected from the group consisting of bariumsulfate, zinc oxide, titanium dioxide, zinc carbonate, US. Cl, X,Rcalcium carbonate, polystyrene beads and glass beads. 15 961; 250-495

